For many years drag sealers, heated wheels and hot air guns have been used to provide a heat seal for moving webs of film. The film is usually formed around a package to form a tube and the lap seal is then made with a heat seal device which can seal the film while it is moving through the packaging machine. Some typical devices consist of drag sealers, heated wheels, hot air guns and heated and cooled belts.
In U.S. Pat. No. 4,218,863 there is shown a continuous motion wrapping machine which utilizes a resistance type air heater in combination with an endless heat sealing belt having perforations which allow for the passage of heated air therethrough to seal the overlapping longitudinal edges of the tubular film. The patent recognizes the problem of changes in the speed of operation of the machine by stating that the temperature of the air may be varied by varying the electrical power supplied to the resistance type air heater or by varying the volume of air through the air heater. While the patent does not further discuss the means of varying of electrical power to the resistance type air heater, it does disclose the use of three solenoid valves which may be operated during different speeds of the machine to allow for selected release of heated air from the hot air nozzle and thus prevent this released hot air from being applied to the film.
This type of hot air sealing device as well as the previously mentioned drag sealers, heated belts and heated wheels provide a seal of consistent quality so long as the three variables of time, temperature and pressure are maintained. Pressure is generally related to the stretch of the film around the product or the pressure exerted by the heated wheel or drag sealer, sometimes with a stationary back up. Where an endless belt is used, pressure may be applied directly to the product. For most applications, this pressure is essentially independent of velocity and temperature and can be regarded as a constant. Temperature is the temperature of the sealing media, i.e., the media which causes the sealing of the film. This can be the temperature of the drag seal or the sealing wheel which is in contact with the film or it can represent the temperature of the heated air which is being blown onto the surface of the film through a hot air gun or through an endless perforated belt. Time is inversely related to velocity. The faster the film runs, the less the sealing time (the time the sealing media is in contact with the portion of the film to be sealed). If it is desired to change the velocity of the film while it is running through a machine, and in order to maintain consistent seal quality it is necessary to modify either time, temperature or pressure to compensate for the change in velocity. Since pressure is a constant the only variable available to compensate for a change in sealing time (velocity) is temperature.
In most applications temperature is regarded as a constant, since it is usually difficult to change the temperature of a heated mass such as a heated wheel or drag seal over a short period of time. With such sealers the seal quality is not consistent with a variable sealing time, i.e. with a variable velocity of the film through the packaging machinery. This is because a change in speed will either produce a burn-through of the seal area caused by the film melting at speeds less than the desired run speed or lack of a good seal at speeds in excess of the desired run speed due to the decreased sealing time. This is particularly true when starting or stopping the equipment or when the equipment is slaved to upstream or downstream equipment with varying production rates which cause variations in the speed of the sealing machine.
Since machines of this type typically run at a wide range of speeds, i.e. 0-300 packages per minute, under varying conditions and generally require a controlled acceleration to get to the desired velocity, seal quality has consistently been a problem especially as it relates to starting and stopping.
The direct result of these limitations would be scrap product produced when machines of this type change speeds to match production conditions. This limitation has been a factor in the development of high speed continuous packaging machines using plastic films.
Hot air has been used for many years to produce seals for these types of packaging and film converting machinery. Hot air systems have distinct advantages over the other systems in that the temperature can be changed very rapidly to match the desired conditions. By contrast most drag sealers or wheels use a considerable amount of thermal mass to accomplish the sealing and as a result they can not be responsive to a rapid change in temperature.
A typical hot air sealing apparatus consists of a quartz heater which produces heat energy as a function of the resistance of the wire used to wind the coils of the heater. The amount of heat produced measured in watts can be expressed as the input voltage times the input current.
By regulating the input power duty cycle with a temperature controller the temperature generated in the coil can be regulated. The temperature controller closes a power loop by sensing the temperature of the discharged air with a sensor such as an RTD or a thermocouple.
The heater requires air to be blown over the coil to remove the heat generated in the coil and keep the coil from burning out. This heat is then directed to the seal area to supply the required energy to seal the package. Since it is desired to maintain a minimum heat for start-up the heat will continue after the film stops and during these stops it is thus often necessary to redirect the heat away from the sealing area and away from the film which is stationary. This may be done by incorporating the heater in a moveable arm which is brought down to the film prior to movement and allowed to retract when the film velocity reaches zero. This procedure can also cause additional problems related to reaction time of the arm as a function of film velocity. Another prior art alternative has been to use an articulated deflection shield to divert the hot air when the machine is stopped. Still another alternative has been to release the hot air through valves as shown in U.S. Pat. No. 4,218,863.
Prior to this invention the control system for hot air seals included a motion controller, a temperature control unit and a programmable controller or PLC used as the coordinator of the system. The motion controller controls the speed of the packaging operation or the velocity of the film passing through the packaging machine. Typically the motion controller operates packaging machinery selectively at several different and discrete predetermined speeds. The temperature controller controls the power to the heating coil and typically there is a high and a low temperature selection for the temperature controller. In coordinating the system, the PLC enables the motion controller determining the speed at which the machine is to be run, and relays or sends the information to the motion controllers. The PLC also controls the status of the temperature controller by enabling the system and energizing either the high temperature set point or low temperature set point based upon the speed being requested of the motion controller by the PLC. The PLC may then coordinate the high speeds programmed into the motion controller with the high sealing temperature programmed into the temperature controller, and coordinate the low speeds programmed into the motion controller with the low sealing temperature programmed into the temperature controller.
In addition, in order to achieve better seal quality, flutes, valves, or other bypasses have been used and are controlled through the PLC as a function of film velocity. The faster the film runs, the more the flutes, valves or other hot air bypasses are closed to direct more hot air to the sealing area. As the speed decreases more flutes, valves, or other hot air bypasses are opened to bypass more hot air away from the sealing area, thus in effect reducing the quantity of heat by bypassing or releasing much of the sealing energy to the atmosphere. Although the bypassing of the sealing energy to the atmosphere at low speeds and the redirecting of the sealing energy to the seal area for high speeds has helped, the seal quality is not usually consistent in seal width or appearance. In addition the effect of opening flutes, valves or other bypasses in accordance with the speed of the film provides a stepped response and does not provide a smooth linear transition from one speed to another, which therefore affects the quality and appearance of the seal.